Method and device for capturing ruthenium present in a gaseous effluent

ABSTRACT

The present invention relates to a method and a device for trapping ruthenium present in a gaseous effluent. The trapping method of the present invention comprises the use of an aqueous solution or slurry of an alkylene glycol polymer or an alkylene glycol copolymer, in which the alkylene(s) has (have) from 2 to 6 carbon atoms, in order to trap ruthenium present in a gaseous effluent. The ruthenium-trapping cartridge of the present invention has a surface (S) on which an alkylene glycol polymer or an alkylene glycol copolymer is placed, in which the alkylene(s) has (have) from 2 to 6 carbon atoms. The present invention makes it possible in particular both to favour the trapping and the chemical operation of reducing ruthenium oxide RuO 4 .

TECHNICAL FIELD

The present invention relates to a method and to a device for trappingruthenium present in a gaseous effluent.

The invention is particularly applicable in the filtration of thegaseous effluents coming from the reprocessing of nuclear fuels thatcontain or are likely to contain ruthenium.

Ruthenium is one of the atomic fission products generated during thenuclear reaction. In this context, it is found in the irradiated fuelrods. It represents 6% by weight of all of the fission products, and itsisotopes ¹⁰³Ru and ¹⁰⁶Ru are radioactive.

In the processes for processing nuclear fuels, the fuel rods are firstlysheared and dissolved in nitric acid. Most of the components making upthe rods, including ruthenium, then pass into solution in the form ofnitrates. This dissolution solution is then sent to liquid/liquidextraction shops. The ruthenium is present at this step of the processin the aqueous phase called the fission product (FP) solution. Thissolution is sent to the vitrification shops where it is calcined in afurnace and the elements in oxide form resulting therefrom are thenvitrified.

Thus, ruthenium, like the other radioelements, is vitrified.Unfortunately, the oxide form RuO₄ is extremely volatile and, althoughtrapped by the treatment carried out on the gaseous effluents comingfrom these processes, a fraction, albeit a minute one, is likely toescape, especially via the possible leaks in the processing circuit.

Ruthenium in this gaseous form RuO₄ can then be transferred into thebuilding ventilation system and pass through the ventilation ducts. Itthen passes through all the filtration barriers of the ventilationsystem. It then gets into the primary stack and is discharged into theenvironment.

At the present time, in most irradiated fuel reprocessing plants, thegaseous effluents coming from the cells emitting ruthenium pass througha set of two filters that strip them of the coarsest particles andprevent too rapid clogging of the following filtration stages. They thenpass through the first and second barrier filters placed in shieldedcontainers.

It is in particular on these filter elements that the present invention,which constitutes a very effective means of preventing the discharge ofruthenium, can preferably be attached.

PRIOR ART

When the ruthenium is in the form of solid RuO₂, it is relatively simpleto trap it using absolute filtration. This is currently the case invitrification shops that possess several filtration barriers in theirventilation systems. The very high efficiency (VHE) filters of thefirst, second and third barriers prevent the passage of solid RuO₂particles. Of course, the VHE filters trap only the RuO₂ that is formedupstream. If the reduction of RuO₄ takes place downstream of the VHEfilters, it is obvious that RuO₂ may be discharged into the environment.

This is because the glass-fibre filter medium of the VHE barriers is notcapable of stopping gaseous RuO₄, which can then pass into the stack,possibly being reduced to RuO₂ in transit. One way of stopping this RuO₄therefore consists in reducing it to RuO₂ upstream of the filtrationbarriers and then in trapping it on a VHE filter.

It is also possible to pass the gaseous effluent containing rutheniumover a reducing medium such as poly(4-vinylpyridine) (PVP) or wet metalsurfaces that act as catalysts. However, solid traps, which areeffective at room temperature, particularly commercially available PVP,generate very substantial head losses and therefore require asignificant increase in the power of the ventilation fans.

It is also possible to carry out a scrubbing operation on the gaseouseffluent by means of an aqueous solution, possibly containing a reactantsuch as sodium hydroxide. However, the carbonation of sodium hydroxideby picking up atmospheric CO₂ requires substantial replenishment of thereactant, and therefore the generation of a large volume of liquideffluent.

In general, the efficiency of these systems proves to be limited. Thisis because the filter elements of the prior art stop most of theaerosols but are incapable of effectively stopping RuO₄.

Obviously from the environmental standpoint there is a real need to havean effective method of trapping ruthenium likely to be present inparticular in the gaseous effluents coming from irradiated nuclear fuelreprocessing plants.

SUMMARY OF THE INVENTION

The inventors have developed a ruthenium trapping method and device thatmeet this need.

In particular, the method of trapping ruthenium present in a gaseouseffluent of the present invention is characterized in that it comprisesbringing the said gaseous effluent into contact with an aqueous solutionor slurry comprising at least one alkylene glycol polymer and/or atleast one alkylene glycol copolymer, in which the alkylene(s) has (have)from 2 to 6 carbon atoms.

The present invention also relates to the use of the aforementionedaqueous solution or slurry for trapping ruthenium present in a gaseouseffluent.

The method of the invention may be employed either in a gas scrubbingunit, the polymer or copolymer then being used as a reactant added tothe scrubbing water, or by manufacturing a ruthenium-trapping cartridge.The said cartridge comprises, for example, a substrate on which analkylene glycol polymer or an alkylene glycol copolymer is placed, inwhich polymer or copolymer the alkylene(s) has (have) from 2 to 6 carbonatoms.

Thanks to the aforementioned polymer or copolymer in aqueous solution,for example used in a gaseous effluent scrubbing unit, the presentinvention makes it possible to achieve, unexpectedly, an efficiencycomparable to that using sodium hydroxide while avoiding theaforementioned carbonation problem.

The scrubbing units that can be used for scrubbing a gaseous effluentusing the method of the present invention are those known to a personskilled in the art. For example, the unit may be a packing column, aventuri scrubber, etc.

When a cartridge is used, the flexibility of the method and of thedevice of the present invention that are based on the aforementionedpolymers and copolymers advantageously makes it possible to designruthenium traps suitable for existing irradiated nuclear fuel processingplants. Furthermore, the amount of polymer that has to be used is verysmall, which really does prevent any safety problem and creates nodifficulty in management of the waste produced by the invention whencarrying out the periodic replacement operations that may be necessary.

The polymer or copolymer may be selected according to the operatingconditions, for example according to the surface temperature, to thenature of the other chemical species present in the gaseous effluent,possibly according to the substrate used, to the cost, to theventilation power, etc. According to the invention, the properties ofchoice of the polymers and copolymers that can be used in the presentinvention may be the following:

the polymer or copolymer is advantageously soluble in water so that itcan be deposited on a substrate by impregnation of aqueous solutions;

the composition of the polymer or copolymer is advantageously simple,for example consisting solely of carbon, oxygen and hydrogen, therebyreducing the costs of the method and the device of the presentinvention; and

the polymer or copolymer is capable of trapping the RuO₄ owing to thefact that it contains one or more reducing groups —OH by analogy withthe reducing effect of sodium hydroxide.

Preferably, the polymer or copolymer has hydroxyl end groups. In thiscase, these are alkylene glycol polymers and copolymers terminated withhydroxyl end groups.

Advantageously, according to the invention, the alkylene glycol polymermay for example be selected from the group consisting of polyethyleneglycol, polypropylene glycol, polybutylene glycol or a blend of these.

Advantageously, the alkylene glycol copolymer is a copolymer consistingof polymers selected from the group consisting of polyethylene glycol,polypropylene glycol and polybutylene glycol. For example, the alkyleneglycol copolymer may be a copolymer based on ethylene glycol, propyleneglycol and butylene glycol at the same time.

Advantageously, according to the invention, the alkylene glycolcopolymer may be of the following formula (I):

in which m and p are integers such that, independently, 1≦m≦8 and3≦p≦12.

The copolymer of formula (I) may for example be a polyethyleneglycol/polypropylene glycol copolymer.

According to the invention, a solution or slurry of an aforementionedpolymer or an aforementioned copolymer alone, of a blend of variousaforementioned alkylene glycol polymers, or of a blend of variousaforementioned alkylene glycol copolymers, or of a blend of one or moreaforementioned polyalkylene glycols and of one or more aforementionedalkylene glycol copolymers may be used in the method and the device ofthe present invention. Also, in the present description, the expression“polymer or copolymer” and the expression “alkylene glycol polymers orcopolymers” cover, of course, these various embodiments of the presentinvention. For trapping ruthenium on a solid substrate, theaforementioned polymers furthermore have the advantage of being able,because of their wetting properties, to be easily deposited as thinlayers on a substrate, thus offering better characteristics in terms ofhead loss and of developed surface area than the products of the priorart.

Thus, when a substrate is used to implement the present invention, theaqueous polymer or copolymer slurry is placed on the substrate. Thisembodiment advantageously makes it possible to reduce the interfacialsurface tension between the substrate and the ambient moisture and thusfavour the trapping of water from the water contained in the gaseouseffluent to be treated on the surface of the substrate, thus making iteasier to absorb the ruthenium and to reduce it.

The forms of ruthenium covered by the present invention are essentiallyRuO₄ and RuO₂. After contact with the substrate, the RuO₄ may beabsorbed by the polymer or copolymer placed on the surface and reactwith the latter. This is because the aforementioned polymers andcopolymers favour the absorption of RuO₄ and limit its desorption, andtherefore allow the RuO₄ to remain on the surface for a long enough timefor it to be reduced. Furthermore, the hydroxyl functional groups ofthese polymers and copolymers reduce this form of ruthenium to RuO₂. Thepresent invention therefore makes it possible both to favour thetrapping of RuO₄ ruthenium and the chemical operation of its reduction.

Also advantageously according to the invention the substrate may bepreferably selected so that it has a large area of contact with thegaseous effluent to be treated for a low head loss. This is because theruthenium present in the effluent comes into contact with the surface bycollision, and it is preferable for the collision factor to be as highas possible so that the maximum amount of ruthenium is trapped. Thus,very preferably, the substrate is a divided substrate, for example asubstrate in the form of fibres, for example a wool or mass of fibres,preferably one that is not compacted when it is desired to avoid headlosses by the flow of the gaseous effluent through said substrate. Afibrous substrate furthermore has the advantage of retaining thepossible solid ruthenium (RuO₂) particles. In the case of such asubstrate, the contacting with the gaseous effluent will advantageouslytake place by forcing the said effluent to pass through the fibroussubstrate.

According to the invention, the substrate may for example be a metalwool, preferably of low density and of highly developed surface area,such as a stainless steel wool. This is because such a substrate makesit possible to achieve a very high efficiency, while generating only avery low head loss, not requiring the existing ventilation fans to bechanged. The substrate may also be a glass wool.

The polymer or copolymer may be placed on the substrate by any suitablemeans known to those skilled in the art. Preferably, for example whenthe substrate is fibrous, this means prevents the substrate from beingclogged so that the gaseous effluent can pass through it, if necessarylimiting the head losses. Advantageously, the polymers and copolymersused in the present invention are soluble in water and therefore allowaqueous solutions, called impregnation solutions, to be prepared, thesebeing practical for placing the polymers or copolymers on the substrate,for example by simply dipping it into the said impregnation solutions.The concentration of the solution will in particular be determinedaccording to the amount of polymer or copolymer to be placed on thesubstrate. The manufacture of this solution and the impregnation aredescribed in the examples below. Preferably, after impregnation, thesubstrate, for example the fibres of which it is composed, will becovered with a thin layer or film of aqueous slurry of the selectedpolymer or copolymer over its entire surface, that is to say, in thecase of fibres, over all its constituent fibres.

According to the invention, the operation of contacting the effluentwith the solution or slurry of the polymer or copolymer, optionallydeposited on a substrate, may be carried out at a suitable temperatureso that the contacted materials (polymers, substrate) are not destroyed.This operation will in general be carried out at a temperature rangingfrom 20 to 50° C.

In the device of the present invention, the cartridge may furthermorecomprise a structure supporting the substrate on which the alkyleneglycol polymer or copolymer is placed. According to the invention, thisstructure, in addition to its function of supporting the said substrate,may be a structure suitable for the insertion of the cartridge into apossibly pre-existing gaseous effluent line. For example, it may be inthe form of a basket. This structure is preferably made of a materialsuitable for its use under the conditions of the present invention, forexample stainless steel. In general, the said structure gives thecartridge its geometry.

According to the invention, the geometry of the said cartridge ispreferably designed so that it can be placed, advantageously in aremovable manner, in a ruthenium-containing gas line so as to force thegaseous effluent to pass through the said cartridge. Thus, this allowsprefabrication of modules, which consist of the substrate and a support,the fitting of which requires no modification of the units nor of theprocedures. Furthermore, the cartridge may be provided with peripheralseals intended to force the said ruthenium-containing gaseous effluentto pass through the said cartridge, preferably without any loss. Thismay be important in order to force the effluent to pass through thepolymer-impregnated or copolymer-impregnated substrate, and avoid anyloss, so as to trap all of the ruthenium present in the effluent in thecartridge.

In a preferred embodiment, the cartridge of the present invention maytherefore comprise:

the substrate on which the alkylene glycol polymer or copolymer isplaced, the said surface being in the form of glass wool or stainlesssteel wool;

a structure, or support, supporting the said substrate on which thealkylene glycol polymer or copolymer has been placed, the said structurepreferably being in the form of a basket, preferably a latticed basket;and

peripheral means for sealing the said cartridge, for example seals, forexample of the type made of Viton (brand name) or silicone, making itnecessary for the gaseous effluent to pass through the said substrate.

According to the invention, one or more cartridges may of course be usedif necessary, for example mounted in series, so that the gaseouseffluent can pass through them in succession.

The ventilation systems involved in the present invention for trappingruthenium are especially those for extraction and for treatment of thevitrification cells, and also those for the cells for dismantlingirradiated nuclear fuel reprocessing plants. The ventilation systems forreprocessing plants are generally composed of several filtrationbarriers:

medium-efficiency (ME) pre-prefilters and high-efficiency (HE)prefilters directly in the cell;

very high-efficiency (VHE) filters for the first and second barriers inshielded containers;

VHE filters for the third barrier in sealed airlock containers; and

HE traps at the base of the stack.

To succeed in trapping the RuO₄, at least one cartridge of the presentinvention may for example be inserted in one or more of theaforementioned filter elements. One embodiment of the present inventionin a plant will be described below in the examples.

A cartridge according to the invention may be positioned either in thefirst barrier or in the second barrier. The filter elements of the firstbarrier will preferably be replaced at least about every two years. Theywill be changed in particular when they become too highly irradiatingowing to trapped radioactive particles, and possibly in the event ofthem being clogged. The filter elements of the second barrier are ingeneral more rarely replaced, as no substantial rise in irradiation orin clogging is observed therein.

Fitting the ruthenium trapping system of the present invention in thefirst barrier has the advantage, should it suffer a loss of efficiency,of benefiting from the periodic changing of this first barrier. However,when installed in this way, the ruthenium trapping substrate or mediumwill undergo more substantial irradiation, liable to accelerate itsageing.

Other features and advantages of the present invention will becomefurther apparent to those skilled in the art on reading the illustrativeexamples that follow, with reference to the appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a test bed used for testing the present invention. Itconsists of a glass tube in which stainless steel discs (S) impregnatedwith polymer or copolymer according to the invention and a unimpregnatedcontrol disc have been placed. A ruthenium-containing gaseous effluentis made to flow through this tube.

FIG. 2 is a graph illustrating the amount of ruthenium retained from theeffluent by being trapped in stainless steel discs impregnated withpolymer or copolymer according to the invention and an unimpregnatedstainless steel control disc.

FIG. 3 is a graph showing the ruthenium trapping efficiency (in % as amonolayer) at room temperature of a layer of WB12 (trade name) substrateimpregnated with polymers differing by their molecular weight (MW), witha degree of impregnation of about 100% by weight.

FIG. 4 is a graph indicating the % trapped ruthenium by weight for eachdisc D1, D2 and D3 of the test bed of FIG. 1, using various polymers andcopolymers according to the present invention.

FIG. 5 is a diagram showing a structure intended to support thesubstrate impregnated with polymer or copolymer according to theinvention. This structure comprises two concentric mesh cylinders.

FIG. 6 is a diagram showing how a substrate impregnated with polymer orcopolymer according to the invention may be placed (in this case wound)around the central cylinder of the structure shown in FIG. 5.

FIG. 7 is a diagram showing how the outer cylinder of the structureshown in FIG. 5 is fitted around the substrate wound around the centralcylinder shown in FIG. 6, in order to form a cartridge according to thepresent invention.

FIG. 8 is a representative diagram of a VHE filter in which thecartridge according to the invention shown in FIG. 7 has been placed.

EXAMPLES Example 1

Example of polymers and copolymers that can be used according to thepresent invention

Grouped together in Table 1 below are various polymers, copolymers andblends that can be used in the present invention.

They are commercially available, for example from:

Lambert Rivière (manufacturer: ICI);

Albright & Wilson;

Roth Sochiel. TABLE 1 Melting point Name Meaning (° C.) PEG Polyethyleneglycol — 300 to Polyethylene glycols having — 35 000 PEG molecularweights ranging from 300 to 35 000 g/mol Copol 1 Polyethylene <0glycol/polypropylene glycol block copolymer Copol 2 Ethyleneglycol/propylene 27 glycol/butylene glycol copolymer Copol 7 PEG 2000 +PEG 300 in 25 proportions of 50/50 by weight Copol 9 PEG 2000 + PEG300 + Copol 1 34 in proportions of 70/20/10 by weight Copol 10 PEG2000 + PEG 300 + Copol 1 23 in proportions of 45/45/10 by weight Copol11 PEG 2000 + Copol 2 in 38 proportions of 50/50 by weight Copol 14Copolymer based on ethylene 37 glycol, propylene glycol and butyleneglycol

Example 2

Example of ruthenium trapping by a copolymer according to the presentinvention

Stainless steel wool (fibre diameter (Ø): 12 μm), called WB12 (tradename), specimens, as substrates, were impregnated with a 5 wt % solutionof a copolymer according to the present invention. The copolymer of thepresent invention, used here, which has surfactant properties, is aPEG/PPG (polyethylene glycol/polypropylene glycol) copolymer, which isliquid at room temperature, denoted in the above Table 1 by Copol 1. Itcomes from Albright and Wilson, with the trade name AMPLICAN.

The operating conditions for the trials were the following:

temperature: 18.5° C.;

relative humidity: 42%;

[O]: 1.8 mg/l;

flow rate: 2.24 m³/h;

duration of the trial: 5 h;

1 unimpregnated disc+3 WB12 discs impregnated 100% with Copol 1.

FIG. 1 shows a test bed (1) used for this example. It consists of aglass tube (2) in which the three WB12 discs (S) 100% impregnated withCopol 1 and the unimpregnated stainless steel control disc (6) have beenplaced. The arrow (8) indicates the direction of flow of theruthenium-containing gaseous effluent through the tube.

The three discs and the upstream control disc of the traps wereanalyzed—the amount of trapped ruthenium (Q_(Ru)) in the discs is givenin Table 2 below. The % trapped Ru corresponds to the amount ofruthenium trapped on a disc relative to the total amount of rutheniumgenerated. The trapped % of Ru impinging on the trap corresponds to theamount of ruthenium trapped on a disc relative to the amount ofruthenium impinging on this disc.

A guard placed downstream of the device allows the amount of Ru nottrapped by the discs to be determined. TABLE 2 Disc 1: Disc 2: Disc 3:Unimpregnated WB12 + WB12 + WB12 + WB12 disc Copol 1 Copol 1 Copol 1Guard Q_(Ru) (mg) 0.056 0.818 0.212 0.014 <0.01 % Ru 5 74 19 1 — trappedTrapped 5 78 94 — — % of Ru impinging on the trap

The results are plotted on the graph in appended FIG. 2. In this figure,“D_(u)” indicates the unimpregnated stainless steel disc and “D1, D2 andD3 ” indicate the various aforementioned discs, in the direction of flowof the gaseous effluent (from D1 towards D3). The results are verysatisfactory since practically all of the ruthenium has been trapped onthe three traps mounted in series.

Example 3

Effect of increasing the melting point of the polymer or copolymeraccording to the invention

To study the impact of a change in melting point of the polymer on itsefficiency, the inventors worked on a series of polymers of the samefamily, for which only the molecular weight and the hydroxyl numbervaried.

These polymers were polyethylene glycols (PEGS) whose characteristicsare given in Table 3 below: TABLE 3 Melting State at room Molecularweight point Hydroxyl temperature (g/mol) (° C.) number I_(OH) PEG 600Liquid   600 15-25° C. 178-197 PEG 1500 Solid   1500 42-48° C. 70-80 PEG35 000 Solid 35 000 60-65° C. 3-4

The graph in FIG. 3 shows the trapping efficiency at room temperature ofa layer of WB12 substrate impregnated with polymer at a level of about100% by weight (polymer mass=stainless steel mass).

The capture efficiency greatly decreases with an increase in molecularweight (MW) and with a reduction in hydroxyl number (I_(OH)). These twoproperties vary inversely with each other—the hydroxyl number is anindicator of the number of polymer chain ends (HO-ether chain-oxide-OH).If a polymer chain is shortened, the number of chain ends (OH) isincreased while, on the other hand, its molecular weight decreases.These parameters are linked in the manner indicated in Table 4 below:TABLE 4 Molecular weight Hydroxyl number PEG 1500 Reduced by a Increasedby a ↓ factor of 2.5 factor of 2.5 PEG 600

The higher the melting point of a polymer is raised, so as to increaseits mechanical strength, the less effective it appears. There istherefore a compromise to be found between mechanical strength andefficiency, which a person skilled in the art would readily be able tofind from the present description.

For the following examples, the inventors have chosen to adopt thepolymers that have a melting point lying within the selected operatingrange of about 40° C.

At this temperature, the polymer is waxy, that is to say non-liquid, inthe form of a soft solid.

Example 4

Polymer Blends

In parallel with seeking a polymer whose melting point is 40°, theinventors produced polymer blends allowing a 40° C. melting point of theblend to be achieved. The basis of the blend was to combine a polymerhaving a high molecular weight and a high melting point with a low-masspolymer which provides it with the surface activity and the hydroxylnumber.

The blends prepared were Copol 7, Copol 11, Copol 2, Copol 9 and Copol10 defined in Table 1 above.

WB12 stainless steel wool was impregnated to an amount of about 100% byeach of these blends, before the test on the test bed described above.The tests were carried out at 20° C. and 40% relative humidity.

The results are given in the graph shown in the appended FIG. 4 whichindicates the % by weight of ruthenium trapped for each disc D1, D2 andD3.

The fact that the efficiency of the layer 3 is greater than that of theupstream layers results from saturation of these upstream layers withRu. All the products tested were very effective, but the selection wasmade based on, as single criterion, lead time constraints and thereforecommercial availability of the reactants.

Since PEG 2000 and Copol 2 were available in sufficient quantity forcarrying out impregnation on an industrial scale, the inventors tookCopol 11 as reference product in this example.

A blend may sometimes have drawbacks, such as demixing, which may resultin the behaviour of the polymer being modified over time. This is why,advantageously, according to the invention, copolymers are preferred andespecially those having all the characteristics of Copol 11 in terms ofmelting point and efficiency. A copolymer having these usefulcharacteristics is, for example, Copol 14, which is a copolymer based onethylene glycol, propylene glycol and butylene glycol, sold for exampleby Lambert Rivière (manufacturer ICI) under the trade name SYMPERONICA20.

Example 5

Impregnation of a Substrate

The impregnation with the copolymer on the substrate is an importantstep in producing the trap cartridge according to the invention. If thisis carried out incorrectly, and especially if the copolymer does notcover all of the substrate, for example all of the stainless steel woolas in this example, the cartridge may let some RuO₄ through and theefficiency of the cartridge will in general be affected. In addition, itis necessary for the impregnation to be homogeneous in order not tocreate preferential paths.

These trials were therefore aimed at controlling the amount of polymeror copolymer deposited on a substrate during the impregnation step.

The first trials consisted in varying the concentration of theimpregnation polymer. The substrate was a WB12 (trade name) stainlesssteel wool. The WB12 stainless steel wool specimens in this example haddimensions of 70×100 mm. They were immersed in the polymer solution andthen placed on a metal (stainless steel) mesh before drying overnight at40° C. The impregnation results are given in Table 5 below: TABLE 5[Copol 14] WB12 Dry WB12 + Copol Degree of (g/l) (g) 14 (g) impregnation100 1.8802 3.8863 107% 50 1.8729 2.7011 44% 25 1.8388 2.2501 22% 101.9469 2.1190 9% 5 2.1222 2.2135 4%

The amount of polymer deposited therefore varied almost linearly withthe concentration of the impregnation solution.

The inventors therefore adopted, by practical choice, a 10 g/limpregnation solution for manufacturing the industrial traps from thiswool.

In the same way, trials were carried out with WB22 (trade name)stainless steel wool. This stainless steel wool differs from WB12 (tradename) by the diameter of the fibres (12 μm in the case of WB12 and 22 μmfor WB22). The weight per unit area of each layer remained the same forboth wools (300 g/m²). The inventors used a 25 g/l impregnation solutionfor this wool.

The impregnation results are given in Table 6 below: TABLE 6 [Copol 14](g/l) Degree of impregnation 40 27% 30 18% 25 10% 20 7.5%  15  6%

The amount of polymer deposited therefore varied almost linearly withthe concentration of the impregnation solution. The 25 g/l concentrationwas used here.

To control the uniformity of polymer deposition on the surface of thesubstrate formed from stainless steel wool (WB12), the inventorssubjected a WB12 disc impregnated with Copol 14 to a stream ofruthenium-containing air.

They observed this specimen under a scanning electron microscope (SEM).They then compared the X-ray image of the specific lines of ruthenium onthe same specimen and showed clearly that these two images weresuperposable and almost identical. This confirmed that the ruthenium wasdeposited uniformly on the surface of the stainless steel wool andtherefore that the polymer covered the stainless steel wool fibresperfectly.

Example 6

Resistance of the Polymer to NOx and to Ozone

Since nitrogen oxides or nitrous vapours (NOx) and ozone are possiblypresent in industrial gaseous effluents, the inventors carried out testson the behaviour of the Copol 14-impregnated support with respect to NOxand ozone.

The Copol family is sensitive to NOx and the reaction results in theformation of degradation products that are unstable and decompose,releasing heat. However, this reaction is neither explosive nor violent.

In the same way as for NOx, stainless steel wool 30% impregnated withCopol 14 was subjected to an ozone stream using, for this, the test beddescribed above. The conditions were defined on the basis of theassumption of ozone generation by radiolysis of air. Specimens weresubjected to a 2.5 m³/h stream of ozonated air with an ozone content of0.7 g/m³ of wet air.

Copol 14 seems to behave in a similar manner with respect to NOx and toozone. However, the ozone-induced degradation phenomena are much lessaccentuated: less heat is generated, exotherms starting at 85° C.

Example 7

Ruthenium trapping device according to the invention

The solution presented in this example made it possible to avoid anymodification of the installations in place. It consisted in placing theruthenium trap of the present invention and the core of a cylindricalVHE filter of the second barrier. This was produced by cutting the upperstrips of the filter and inserting a basket containing Copol14-impregnated WB12 wool.

Copolymer:

The copolymer selected in this example was Copol 14 (see Table 1).

Substrate

The substrate selected was a stainless steel wool because this offered alarge contact area with the gaseous effluent for a lower head loss. Thestainless steel wool WB12 (trade name) is composed of stainless steelfibres with a diameter of 12 microns. Its specific surface area is 13m²/m² for a wool 7 mm in thickness, i.e. about 1857 m²/m³ ofnon-compacted wool. Its weight per unit area is 300 g/m², i.e. about 43kg/m³ (again not compacted).

Impregnation

Several impregnation techniques were tested with the objective ofimpregnating the trap entirely; basket+2 kg of stainless steel wool.After many trials, it was decided to impregnate, sheet by sheet,stainless steel wool and to assemble the trap as follows. The intendeddegree of impregnation was 5% using the method of immersing the sheetsof stainless steel wool.

A quality criterion was set in this experiment, this consisting indiscarding any sheet whose degree of impregnation was less than 2% orgreater than 10%. Thus, for a trap containing about 2 kg of stainlesssteel wool, the maximum amount of Copol 14 was 200 g. The impregnationsolution used was 10 g of copolymer per litre of water (see the exampleabove). The impregnated wool was dried flat at 40° C.

The Cartridge

The basket-type metal support of the trap cartridge had the shape of adouble cylinder, as shown in the appended FIG. 5, namely an internalcylinder (C_(i)) and an external cylinder (C_(e)). The internal cylinder(C_(i)) was made of perforated C10U12 stainless steel sheet, i.e.perforated with holes of 10 mm² and a centre-to-centre distance of 12 mm(mesh). This cylinder was welded to a circular base (B_(c)) made of astainless steel sheet of larger diameter, with a hole at its centre inorder to allow passage of a shaft for supporting the filter element (ifsuch a cylinder is needed; a support with no hole at its centre is ofcourse possible).

Eight layers of copolymer-impregnated stainless steel wool were woundaround the first cylinder (C_(i)), which layers formed the substrate (S)as shown in FIG. 6. Two additional layers of stainless steel wool, notimpregnated with copolymer, were then added on top. The externalcylinder (C_(e)) covered the stainless steel wool and comprised astainless steel mesh measuring 12.7×12.7. The base of the support was aflat bottom made of stainless steel, with a hole so as to allow passageof the shaft for supporting the VHE filter element. The trap cartridgetherefore possessed in total 10 layers of stainless steel wool. The twolayers wound last were not impregnated, that is to say contained nocopolymer. They prevented any migration of the impregnation copolymer tothe outside of the element.

The cartridge (CA) obtained according to the invention is shown in FIG.7. Its total mass, consisting of the basket+wool+copolymer, was about 8kg distributed approximately in the following manner:

-   -   basket structure: about 5.5 kg;    -   stainless steel wool: between 2 and 2.5 kg;    -   copolymer deposited: 200 g (maximum);    -   Viton (trade mark) and silicone seals (idem VHE): 300 to 400 g.        Insertion of the cartridge according to the invention into an        existing unit

To conclude, the trap cartridge manufactured according to the inventionwas inserted inside a VHE filter element consisting of glass fibres (F)supported by a perforated sheet (T_(p)). The whole assembly is shown inthe appended FIG. 8.

The supporting shaft (Ax) was therefore removed from the filter element(F) and the trap cartridge (CA) slid onto the inside of it. A siliconeseal (J) was then applied at the ends of the trap cartridge in order toensure adhesion and sealing between the trap cartridge and the filterelement (F). The support shaft was then put back into place. The filterelement and its trap cartridge were ready to be fitted into the shieldedcontainers of vitrification shops.

Head Loss Measurements

The head loss measurements were carried out on this assembly for variousflow rates of gas to be treated. They are given in Table 7 below.

These values were measured on several trials to ±25%.

The impregnation of the stainless steel wool with the copolymertherefore had, in the present case, no significant effect on the headloss. TABLE 7 Flow rate m³/h 1000 2000 3000 Head loss in the VHE filterPa 90 180 270 element Head loss in the trap Pa 40 100 200 cartridgesupport Head loss in the 10 Pa 110 200 360 stainless steel wool layersTotal Pa 240 480 830

Example 8

Active Agent Trial

An experimental loop comprising, in this order; one or two experimentalcartridges in series (Exp. 1 and Exp. 2) according to the presentinvention, no or one PVP cartridge, a filter paper and two PVPcartridges in series (PVP1 and PVP2), a volumetric counter and a pumpwere manufactured. The gaseous effluent passed through this loop in theabove order. The diameter of the cartridges was 5 cm. The draw-off ratesallowed flow speeds (empty drum) of 0.5 to 1 m/s to be achieved, thesebeing representative of the flow speeds in the 2nd barrier VHE filtersof existing irradiated fuel reprocessing plants.

The device was fitted in a vitrification shop, downstream of thefilters.

A first series of trials was carried out on a 100% Copol 1-impregnatedglass wool. The results are given in Table 8 below, in which ¹⁰⁶Ru.Rh(Bq) represents the amount of ruthenium (and its descendent, rhodium)measured by radiometry. TABLE 8 ¹⁰⁶Ru.Rh (Bq) Exp. 1 PVP Filter paperPVP1 PVP2 Volume (m³) 1 week 320 6.2 <5.7 <8.6 <7 620 2 weeks, 410 4.94.2 <7.7 <6.7 >200 new cartridge

Over one week of operation, the results were encouraging, the PVP justdownstream of the experimental cartridge being at the limit ofdetection, indicating that no leakage had taken place.

A second series of trials was carried out on WB12 stainless steel woolimpregnated with Copol 1 to 100%. The cartridge consisted of 8 layers ofWB12. It was left in place for an endurance test.

The results are given in Table 9 below: TABLE 9 ¹⁰⁶Ru.Rh (Bq) Exp. 1 PVPFilter paper PVP1 PVP2 Volume (m³)  7 days ns <6.3 5.4 5.5 <7.8 533 14days ns <7.8 6.2 5.8 <6.1 309 21 days ns 18 9.2 16 <6.3 359 31 days 6500600 160 1100 40 1226 38 days 5900 2500 <6.8 <8.2 17 396(ns: not sampled)¹⁰⁶Ru.Rh (Bq) represents the amount of ruthenium (and its descendent,rhodium) measured by radiometry.

After 21 days testing, the inventors suspected a leak and the cartridgewas removed 7 days later. The results on the downstream PVP, and therepositioning of the cartridge for 7 days, confirmed this leak, whichwas due to slow migration of the Copol 1, this product being too fluidunder the test conditions (40° C.).

A third series of trials consisted in evaluating the efficiency of aWB22 stainless steel wool impregnated with Copol 2 sold for example byLambert Rivière (manufacturer: ICI), under the trade name SYMPERONICAll. It was used in an amount of 22%. The cartridge consisted of asingle layer of WB22. The results are given in Table 10 below: TABLE 10¹⁰⁶Ru.Rh (Bq) Empty Exp. 1 cartridge Filter paper PVP1 PVP2 Volume (m³)7 days 4000 110 74 92 7.5 1054

A single layer already proved to be very effective, despite a flow speedfrom 2 to 3 times higher than during the previous trials.

The fourth trial was an endurance trial in a configuration similar tothat used for the second barrier traps, namely 8 layers of WB12impregnated with 5.7% Copol 14 (these 8 layers were distributed over 2cartridges (Exp. 1 and Exp. 2), i.e. 8 cm in thickness). The results aregiven in Table 11 below: TABLE 11 ¹⁰⁶Ru.Rh (Bq) Exp. 1 Exp. 2 Filterpaper PVP1 PVP2 Volume (m³) *9 days   8600 280 57 71 25 858 20 days nsns 3.7 ns 5 970 25 days ns ns 4.1 <7.4 <7.1 369 32 days ns ns <5.4 <8.4<4.9 621 39 days ns ns <7 <9.6 <8.2 596 49 days ns ns <7.2 <4.2 <8.8 72356 days ns ns <4.7 <3.1 <4.4 528 63 days ns ns <5 <6.5 <7 / 70 days nsns 6.7 <7 <6.9 586 79 days ns ns <6 <7.2 <7 759 86 days ns ns <6.2 <4.2<6.8 504 93 days ns ns <5.2 <7.4 <8.7 539 100 days  ns ns <4.7 <5.9 <7561 109 days  ns ns 17 8 <8 830 118 days  ns ns 26 <7.9 <7.4 563 124days  ns ns 22 <9.2 <6.5 443 133 days  ns ns 18 <8.7 <6.8 619 140 days ns ns 10 <7.4 <7.4 585 144 days  34 000 430 <8.1 <7.4 <7.5 320ns: not sampledAfter 144 days of the trial, corresponding to the treatment of 11 790m³, the experimental cartridges were removed without any lowering ofefficiency being observed.*A sealing fault was identified, manifested by a slight activity on thePVPs. The cartridges were removed in order to fit seals and were countedbefore being reinstalled.

Example 9

Ruthenium Trapping by an Aqueous Copolymer Solution

According to the invention, the ethylene glycol, propylene glycol andbutylene glycol polymers and copolymers can be used as reactants addedto the scrubbing water in a gas scrubbing unit (packing column, venturi,etc.).

Specifically, comparative trials were carried out with variousreactants, in which RuO₄-laden air flowed over the surface of theliquid. The physical parameters (geometry and air speed) were the samefor all the trials, only the chemical composition for the solutionvarying.

The results given in Table 12 below show, for example, that an ethyleneglycol/propylene glycol copolymer, called here Copol 1, is veryeffective for absorbing RuO₄. TABLE 12 pH RuO₄ RuO₄ (measured generatedabsorbed % or (10⁻⁶ mol) (10⁻⁶ mol) absorbed calculated) Trials withpure water Pure water: 19.04 6.02 31.6 5.7 Trial A Pure water: 9.80 3.2132.8 5.7* Trial B Trials in the presence of reactants Na₂CO₃(0.4M) +19.73 4.02 20.4 10.1 NaHCO₃ (0.2M) Na₂CO₃(0.4M) + 9.43 2.00 21.2 9.5NaHCO₃ (0.2M) Buffer (pH = 7) 3.83 1.04 27.2 6.9 Na₂SO₄ 9.48 2.64 27.87.5 HNO₃ 5.31 1.76 33.1 1.6 0.01M sodium 21.69 7.13 32.9 — hydroxide0.1M sodium 11.92 6.66 55.9 hydroxide: Trial A 0.1M sodium 23.61 13.3756.6 hydroxide: Trial B 1M sodium 11.69 11.17 95.6 hydroxide: Trial A 1Msodium 12.11 10.1 83.4 hydroxide: Trial B 0.5M NHA 3.79 3.06 80.70.0475M NHA 7.15 6.25 87.5 5% Copol 1 13.28 13.1 98.6

1. Use of an aqueous solution or slurry comprising at least one alkyleneglycol polymer and/or at least one alkylene glycol copolymer in whichthe alkylene(s) has (have) from 2 to 6 carbon atoms, for trappingruthenium present in a gaseous effluent.
 2. Use according to claim 1, inwhich the alkylene glycol polymer is selected from the group consistingof polyethylene glycol, polypropylene glycol, polybutylene glycol, or ablend of these.
 3. Use according to claim 1, in which the alkyleneglycol copolymer is a copolymer consisting of polymers selected from thegroup consisting of polyethylene glycol, polypropylene glycol andpolybutylene glycol or a blend of these.
 4. Use according to claim 1, inwhich the alkylene glycol copolymer is a copolymer based on ethyleneglycol, propylene glycol and butylene glycol.
 5. Use according to claim1, in which the alkylene glycol copolymer is of the following formula(I):

in which m and p are integers such that, independently, 1≦m≦8 and3≦p≦12.
 6. Use according to claim 1, in which the alkylene glycolcopolymer is an ethylene glycol/propylene glycol copolymer.
 7. Useaccording to claim 1, in which the aqueous solution or slurry is placedon a substrate made of fibres.
 8. Use according to claim 7, in which thesubstrate consists of a glass wool or a stainless steel wool.
 9. Useaccording to claim 7, in which the alkylene glycol polymer or thealkalene glycol copolymer is placed on the said substrate by dipping thesubstrate into an aqueous solution of the said polymer or of the saidcopolymer.
 10. Use according to claim 1, in which the alkylene glycolpolymer or alkylene glycol copolymer has hydroxyl end groups. 11.Ruthenium-trapping cartridge, the said cartridge comprising a substrateon which an alkylene glycol polymer or an alkylene glycol copolymer isplaced, in which the alkylene(s) has (have) from 2 to 6 carbon atoms.12. Cartridge according to claim 11, in which the alkylene glycolpolymer is selected from the group consisting of polyethylene glycol,polypropylene glycol and polybutylene glycol.
 13. Cartridge according toclaim 11, in which the alkylene glycol copolymer is a copolymerconsisting of polymers selected from the group consisting ofpolyethylene glycol, polypropylene glycol and polybutylene glycol. 14.Cartridge according to claim 11, in which the alkylene glycol copolymeris a copolymer based on ethylene glycol, propylene glycol and butyleneglycol.
 15. Cartridge according to claim 11, in which the alkyleneglycol copolymer is of the following formula (I):

in which m and p are integers such that, independently, 1≦m≦8 and3≦p<12.
 16. Cartridge according to claim 11, in which the alkyleneglycol copolymer is an ethylene glycol/propylene glycol copolymer, of acopolymer.
 17. Cartridge according to claim 11, in which the substrateconsists of fibres.
 18. Cartridge according to claim 11, in which thesubstrate consists of a glass wool or a stainless steel wool. 19.Cartridge according to claim 11, in which the said alkylene glycolpolymer or the said alkylene glycol copolymer is placed on the saidsurface by dipping the substrate into an aqueous solution of the saidpolymer or of the said copolymer.
 20. Cartridge according to claim 11,the said cartridge furthermore including a structure that supports thesubstrate on which the alkylene glycol polymer or copolymer is placed.21. Cartridge according to claim 11, comprising: the substrate on whichthe alkylene glycol polymer or copolymer is placed, the said surfacebeing in the form of glass wool or stainless steel wool; a structuresupporting the said substrate on which the alkylene glycol polymer orcopolymer has been placed; and peripheral means for sealing the saidcartridge, making it necessary for the gaseous effluent to pass throughthe said substrate.
 22. Use according to claim 1, in which the aqueoussolution is added to the scrubbing water of a gas scrubbing unit.